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BACKGROUND OF THE INVENTION
This invention relates to measurement devices and more particularly to a
device for use in measuring water depth on the surface of a pathway, such
as an airport runway or the like.
Easily ascertainable and accurate knowledge of the depth of water
accumulating on the surface of an airport runway is very useful and in
some situations a necessity. At airports, for example, decisions, during a
rainstorm, as to which runways are safe to use and which are not, affect
hundreds of lives. The ability of the airport operations manager to
discern, at a glance, on which runways or portions thereof the danger of
hydroplaning exists enables him to determine which runways to close and
which to continue in use.
To be effective, a water depth measuring system must employ a sensing
device which is easily installed at remote locations and which is capable
of withstanding the constant passage of airplanes and vehicles. Further,
the device must be capable of developing accurate information. In
coassigned U.S. Pat. No. 3,882,381 and 3,873,927 sensors and a system are
disclosed for reliably determining and indicating when water or ice are
present on the surface of an airport runway or highway. However, these
sensors and the systems of these patents were not intended for nor do they
provide reliable water depth sensing and indication.
SUMMARY OF THE INVENTION
Among the several objects of the invention may be noted the provision of a
device which reliably and accurately determines the depth of water
accumulating on the surface of pathways such as an airport runway; the
provision of such a sevice which is easily embedded in the surface of a
pathway for exposure of the device to atmospheric precipitation; the
provision of such a device which is rugged in use; the provision of such a
device for providing a monotonic output signal for a preselected range of
water depth; the provision of such a device in which erroneous water depth
indications are avoided by precluding any water depth indications if water
on the surface of the pathway contains contaminants; and the provision of
such a device which is high in sensitivity and reliability, low in cost
and compact in size.
Briefly, a device for use in determining the depth of water accumulating on
the surface of a pathway, such as an airport runway or the like, comprises
a block of electrically insulative material embedded in a pathway with the
top surface of the block being substantially flush with the pathway
surface and exposed to atmospheric precipitation. A sensor comprising
first and second capacitors each of which has first and second coplanar
spaced apart electrodes, is positioned within the block with the
electrodes of the first capacitor positioned adjacent to and inclined with
respect to the top surface of the block and with the electrodes of the
second capacitor being positioned a substantially greater distance from
the top surface of the block whereby an accumulation of water on the
surface of the block affects the dielectric, capacitance and conductance
of the first capacitor without substantially affecting the dielectric,
capacitance and conductance of the second. Means are provided for applying
time-varying out-of-phase electrical signals to the first electrodes of
the aforesaid capacitors as the input to the sensor. Output means are
commonly connected to the second electrodes of the aforesaid capacitors
for supplying an electrical output signal relative to electrical ground
from the sensor. At least one electrical conductor within the block is
connected to electrical ground with a portion thereof exposed to
atmospheric precipitation at the top surface of the block. Accumulation of
water on the surface of the block affects te capacitance and conductance
between the output means and electrical ground whereby the amplitude of
the electrical output signal of the sensor is a function of the depth of
water accumulating on the surface of the pathway.
Other objects and features will be in part apparent and in part pointed out
hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective of a device of the present invention;
FIG. 2 is a sectional view of the device of FIG. 1 on line --2;
FIG. 3 is a sectional view of the device of FIG. 1 on line 3--3;
FIG. 4 is a schematic diagram of a system for measuring water depth using
the device of the present invention; and
FIG. 5 is a graph illustrating the voltage output of a device of the
present invention versus water depth both before and after the inclination
of the first sensor and the addition of the grounding pins.
Corresponding reference characters indicate correponding parts throughout
the several views of the drawings.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring now to the drawings, a device for use in determining the depth of
water accumulating on the surface of a pathway, such as an airport runway
or the like, is generally indicated at 1. Device 1 has a block 3 of
electrically insulative material which is embedded in the pathway so that
top surface 5 of block 3 is substantially flush with the surface of the
pathway and exposed to atmospheric precipitation. The material used in
forming block 3 is preferably a synthetic resin material, such as an epoxy
or the like, and is dark colored.
A sensor 7 comprising first and second capacitors C1 and C2 is positioned
within block 3. Capacitor C1 has coplanar, spaced apart, interdigitated
copper electrodes C1F and C1S which are etched on the top surface 9 of an
electrically insulative substrate or printed circuit board 11. Capacitor
C2 also has coplanar, spaced apart interdigitated copper electrodes C2F
and C2S which are etched on the opposite or bottom surface 13 of printed
circuit board 11. Electrodes C2F and C2S of capacitor C2 are positioned on
bottom surface 13 of board 11 so as to be parallel to and in registry with
electrodes C1F and C1S of capacitor C1.
Sensor 7 is positioned in block 3 with electrodes C1F and C1S of capacitor
C1 adjacent to and inclined with respect to top surface 5 of block 3. More
specifically, sensor 7 is so inclined with respect to top surface 5 of
block 3 that electrode C1F of capacitor C1 is nearer top surface 5 of
block 3 than is electrode C1S. Thus, electrodes C2F and C2S of capacitor
C2 are positioned a greater distance from top surface 5 of block 3. As
shown in FIG. 2, printed circuit board 11 is encapsulated in block 3 with
one end of top surface 9 of board 11 nearer top surface 5 of block 3 than
the other end. As a consequence, accumulation of water on top surface 5 of
block 3 affects the dielectric, capacitance and conductance of capacitor
C1 without substantially affecting the dielectric, capacitance and
conductance of capacitor C2.
Device 1, when embedded in a pathway surface, is interconnected to a remote
monitoring station through a cable 15. Cable 15 has a plurality of shield
electrical conductors three of which (17, 19 and 21) are shown in FIG. 2.
The shielding of these conductors is connected to electrical ground at a
convenient location preferably at the monitoring station. Conductor 17 is
connected to electrode C1F of capacitor C1, and conductor 19 is connected
to electrode C2F of capacitor C2. Conductor 21 is connected between an
output of device 1 and an amplifier A1, which may be located at the remote
monitoring station.
Referring to FIG. 4, a signal generator 23 having, for example, a 200 KHZ
frequency output is connected between electrical ground and a junction
point 25. Signal generator 23 is a conventional signal generator having
the usual gain control to vary the amplitude of its output signal. A
time-varying electrical signal from generator 23 is supplied, via
conductor 17, to electrode C1F of capacitor C1. This signal is also
inverted by an inverter 27 and supplied, via conductor 19, to electrode
C2F of capacitor C2. A potentiometer P1 is provided at the input to
inverter 27 to adjust the amplitude of the 180.degree. out-of-phase signal
applied to electrode C2F for reasons to be discussed. The out-of-phase,
time-varying electrical signals thus applied to electrodes C1F and C2F are
the input to sensor 7.
Electrode C1S of capacitor C1 and electrode C2S of capacitor C2 are
commonly connected at a junction point 29 as shown in FIG. 4. A
time-varying electrical signal developed at junction point 29 is supplied
oh conductor 31 as the electrical output signal of sensor 7, the amplitude
of this signal being relative to electrical ground. The common connection
between the second electrodes of capacitors C1 and C2 is obtained by
drilling a hole through printed circuit board 11 and inserting through
this hole an electrical conductor which is soldered to the second
electrode of each capacitor. As shown in FIG. 1, an electrical conductor
33A is positioned within block 3 of electrically insulative material.
Conductor 33A, which is preferably a metallic pin, is connected to
electrical ground by a grounding wire 35, and a portion of conductor 33A
is exposed to atmospheric precipitation at top surface 5 of block 3. The
presence of conductor 33A creates a capacitance and a conductance effect
between the output of sensor 7 and electrical ground. This shunting effect
is analogous to placing, in parallel, a variable capacitance and a
variable resistance between junction point 29, the output of sensor 7, and
electrical ground.
Additional electrical conductors 33B through 33H are positioned within
block 3 to enhance the coupling and conductance effects between junction
point 29 and electrical ground. Each of these conductors is connected to
electrical ground by grounding wire 35. Further, each has a portion
exposed to the atmosphere at top surface 5 of block 3. As shown in FIGS. 1
and 3, the exposed portion of electrical conductors 33B through 33H are
laterally spaced from and positioned about sensor 7. Preferably, a portion
of the electrical conductors, 33E through 33H, are positioned between the
first and second interdigitated electrodes of capacitor C1 and are
insulated from the electrodes by the insulative material of printed
circuit board 11. Since all electrical conductors extend through printed
circuit board 11, as shown in FIGS. 2 and 3, conductors 33E through 33H
are also positioned between the first and second interdigitated electrodes
of capacitor C2 and are similarly insulated. The inclusion of electrical
conductors 33A through 33H in block 3, along with the inclination of
sensor 7 with respect to top surface 5 of block 3, results in the output
voltage of sensor 7 having a monotonic relationship with the depth of
water accumulating on the surface of the pathway over a preselected range
of water depth.
Referring to FIG. 5, curve A represents the relationship between the output
voltage of sensor 7 and the depth of water accumulating on the surface of
the pathway. For a range of water depth of zero to 0.10 inches, curve A is
single valued or monotonic; that is, a single value of output voltage
obtained from sensor 7 represents only one water depth. curve B represnets
the relationship between water depth and the output voltage of a sensor
disclosed in coassigned U.S. Nos. 3,882,381 and 3,873,927 which does not
include electrical conductors 33A through 33H nor the inclination of the
sensor with respect to the top surface of the block. For the range of
water depth plotted in FIG. 5, curve B is bivalued and therefore not
monotonic; that is, for a portion of the range of water depth shown, one
output voltage value obtained from the prior disclosed sensor can
represent more than one water depth.
Device 1 is useful in a system for warning when hydroplaning conditions are
present on the surface of a runway. To provide such warnings, the
electrical output signal from sensor 7 is converted to a d.c. voltage.
When the d.c. voltage amplitude exceeds a predetermined level, a signal is
supplied to illuminate a warning light signifying the presence of
dangerous conditions.
The electrical input signal supplied to electrode C1F of capacitor C1, in
the manner previously discussed, is coupled to electrode C1S. Similarly,
the 180.degree. out-of-phase electrical input signal supplied to electrode
C2F of capacitor C2 is coupled to electrode C2S. Since electrodes C1S and
C2S are commonly connected, a signal is developed at the output of sensor
7, junction point 29, which is a composite of the electrical signals
coupled between the first and second electrodes of the respective
capacitors. In the absence of precipitation, the amplitude of the signal
developed at junction point 29 is zero. Because of the different
thicknesses of electrically insulative material covering the electrodes of
capacitors C1 and C2, as shown in FIG. 2, the capacitance of capacitor C1
differs only slighly from that of capacitor C2 and this effects the degree
of coupling between the electrodes of the respective capacitors. In order,
therefore, to obtain a zero amplitude output signal at junction point 29,
under clear surface conditions, potentiometer P1 is adjusted to compensate
for the effect of this minor difference in capacitance by adjusting the
amplitude of the signal applied to electrode C2F.
When rainwater is present on top surface 5 of block 3, the dielectric,
capacitance and conductance of capacitor C1 increases while that of
capacitor C2 remains unaffected. The resultant electrical signal developed
at junction point 29 no longer has zero amplitude but rather an amplitude
which is a function of the depth of water accumulating on the runway
surface. The electrical output signal from sensor 7 is supplied on line 31
to the input of an alternating-current to direct-current converter 37
which is preferably constructed on bottom surface 13 of printed circuit
board 11. The output of converter 37 is a d.c. voltage the amplitude of
which is representative of the depth of water accumulating on the surface
of the runway. The d.c. voltage output of converter 37 is supplied, via
shielded electrical conductor 21, to one input of amplifier A1. The gain
of amplifier A1 is adjusted by the setting of a potentiometer P2 connected
in the feedback loop of the amplifier.
The d.c. output voltage of amplifier A1 is supplied to a voltage divider
network which includes resistors R1 and R2 and a potentiometer P3.
Potentiometer P3 is adjusted so that the input voltage to a comparator 39
does not exceed a predetermined level until a predetermined depth of
water, for example 0.05 inches, is present on the surface of the runway.
When the predetermined depth of water has accumulated on the runway
surface, the input voltage to comparator 39 exceeds the predetermined
level, and the output of comparator 39 becomes a voltage which is supplied
to the base of an NPN transistor T1 through a resistor R3. Transistor T1
is turned on, and a ground path is provided through signal lamp L1 and
transistor T1 for a supply voltage +V. Signal lamp L1 is illuminated and a
visual indication or warning is provided. Turn-on of transistor T1 also
causes turn-on of a PNP transistor T2. If switch S1 is closed, turn-on of
transistor T2 results in supply voltage +V being supplied to a buzzer or
audio alarm circuit 41 and an audible alarm is provided along with the
visual warning.
The d.c. output voltage from amplifier A1 is also supplied to a second
voltage divider network including resistors R4 and R5 and a potentiomenter
P4. Potentiometer P4 is adjusted so that the input voltage to a comparator
43 does not exceed a predetermined level until a second predetermined
depth of water, for example 0.10 inches, is present on the surface of the
runway. When the second predetermined depth of water has accumulated on
the runway surface, the input voltage to comparator 43 exceeds the
predetermined level and the output of comparator 43 becomes a voltage
which is supplied to the base of an NPN transistor T3. Turn-on of
transistor T3 provides a ground path for supply voltage +V through a
signal lamp L2, a lamp flashing circuit 45 and the transistor. Signal lamp
L2 now flashes on and off providing a second visual indication or warning.
It is to be understood that circuitry may be provided by which the
illumination of signal lamp L2 would also cause signal lamp L1 to be
turned off.
Referring again to FIG. 1-3, a conductivity sensor 47 is shown attached to
and embedded in block 3. Sensor 47 has first and second spaced apart
electrodes 49 and 51, electrode 51 being mounted to the side of printed
circuit board 11 and electrode 49 being positioned parallel with electrode
51. the gap between electrodes 49 and 51 is filled with an insulative
material. As shown in FIG. 3, both electrodes, which are preferably strips
of stainless steel, are exposed to atmospheric precipitation at top
surface 5 of block 3. Further, electrode 51 is connected to electrical
ground by a grounding wire 53.
The presence of contaminants, for example salt, in the water accumulating
on top surface 5 of block 3 is sensed by conductivity sensor 47. Because
the presence of contaminants adversely effects the accuracy with which
water depth is sensed using device 1, the response of conductivity sensor
47 is used to preclude water depth indications and thus prevent erroneous
ones. Preclusion is effected by producing a voltage whose amplitude is a
function of the conductance of sensor 47, comparing the amplitude of that
voltage with the amplitude of a reference voltage representative of the
conductance of sensor 47 when water containing contaminants is present on
surface 5 of block 3 and disabling visual indications when the water
contains contaminants.
Referring to FIG. 4, a signal generator 55 supplies an a.c. voltage, via
shielded electrical conductor 57, to the ungrounded electrode 49 of
conductivity sensor 47; the sensor 47 and resistor R6 constituting a
voltage divider. The resulting a.c. voltage developed across conductivity
sensor 47 is supplied to the input of an amplifier A2. The a.c. voltage
output of amplifier A2 is supplied to the input of a second amplifier A3
whose voltage output is rectified by a half-wave rectifier circuit which
includes diodes D1 and D2, resistors R7 and R8 and a capacitor C3. The
rectified voltage output of amplifier A3 is a d.c. voltage the amplitude
of which is a function of the conductance of sensor 47. This voltage is
supplied to one input of a comparator 59. The other input to comparator 59
is a reference voltage whose level is determined by adjusting a
potentiometer P5. This reference voltage level, as noted, represents the
conductance of sensor 47 when water containing contaminants is present on
the surface of sensor 47.
The output of comparator 59 is a voltage which is supplied to the base of
an NPN transistor T4. The output voltage of comparator 59, is, for
example, greater than 0.75 volts when uncontaminated water is present on
conductivity sensor 47. For this condition, transistor T4 is off and the
voltage output of amplifier A1, the voltage whose amplitude is
representative of water depth, is supplied to the water depth indication
circuitry. If, however, the water on the surface is contaminated, the
conductance of sensor 47 is such that the output voltage of comparator 59
decrease and, example, falls to 0.25 volts. For this condition, transistor
T4 is turned on and the output of amplifier A1 is grounded through
transistor T4 thereby precluding any voltage being supplied to the water
depth indication circuitry. This condition continues so long as water
containing contaminants is present on sensor 47.
It is to be understood that the water depth measuring device and circuitry
described herein may be incorporated in the ice and water detector systems
of the aforesaid U.S. patents and thus supply additional information,
i.e., the depth of water existing at any time on the pathway.
In view of the above, it will be seen that the several objects of the
invention are achieved and other advantageous results attained.
As various changes could be made in the above constructions without
departing from the scope of the invention, it is intended that all matter
contained in the above description or shown in the accompanying drawings
shall be interpreted as illustrative and not in a limiting sense.
* * * * *
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